1. Field of Invention
The invention relates in general to an encapsulation method of an organic electroluminescent (OEL) device, and more particularly to a method of encapsulating the OEL device by using nitride or carbide.
2. Description of Related Art
It has been over thirty years since the study of the OEL first started in the 1960s. In 1963, a research using a crystalline organic compound to illuminate by applying a voltage as high as 400 Volts was published. However, the intensity of the luminescence is too weak for the demand of application.
In 1987, Kodak Company announced an OEL device with a high intensity of luminescence. The practical application of the organic electroluminescent device thus causes a great interest and an intensive study in industry, government, and academia. The current development and research of the organic electroluminescent device are more directed towards the structure and material of the device.
One of the characteristics of an OEL device is using organic compound, such as tri-(8-hydroxyquinoline) aluminum (Alq.sub.3), to form the luminescent layer. A hole injection layer is inserted between the luminescent layer and a cathode to increase the recombination efficiency of electrons and holes, so that an organic electroluminescence is obtained. With the characteristics including self-luminescence, wide visual angle (up to 160 degree), high response speed, low driving voltage, and true colors, the OEL is thought as the technology of plat panel display for the next generation. Currently, the development of OEL device almost reaches the stage of practical application stage. It is expected that the OEL can be applied to the flat panel display of the next generation. The planar luminescent device is greatly applied in true color planar display device, such as small display panel, outdoors display billboard, computer monitor, TV etc.
The organic film formed in the OEL device is very sensitive to moisture, oxygen and temperature. Moisture and oxygen deteriorate the structure of the material, while a high temperature destroys the conformation of the material. These three factors affect the lifetime of the OEL device seriously.
To prevent the lifetime of the OEL device from being shortened by moisture, oxygen, or high temperature, an encapsulation layer is formed.
FIG. 1 is the cross-sectional view of an OEL device after being encapsulated. An Indium-Tin-Oxide (ITO) layer 102 is formed on a glass substrate 100. An ITO glass substrate is thus formed by the ITO layer 102 and the substrate 100. An organic electroluminescent layer 104 is formed on the ITO layer 102 by thermal evaporation. A metal electrode 106 is formed on the OEL layer 104.
Epoxy resin is used as the encapsulation material. An epoxy resin layer 108 is formed along the periphery of the electroluminescent layer 104 and the electrode 106. A glass layer 110 is further formed to cover the top surface of the electrode 108 to accomplish the encapsulation process. The major drawback of the method as shown in FIG. 1 is the encapsulation can not isolate the electroluminescent device from being deteriorated by the penetration of oxygen and moisture.
In semiconductor manufacture, silicon nitride (SiN.sub.x) is a commonly used dielectric material. Silicon nitride is often applied as a mask while etching or fabricating a filed oxide, or as a passivation layer for semiconductor packaging. Typically, a silicon nitride layer is deposited by plasma enhanced chemical vapor deposition (PECVD) at a reacting temperature range of 200 to 400.degree. C. This temperature range is so high that the structure of the temperature-sensitive OEL layer is very likely to be damaged or destroyed. Therefore, silicon nitride made by PECVD at the above temperature cannot be applied as the encapsulation process for the OEL device. Silicon carbide has a very similar property to silicon nitride, thus, it is not a proper process for encapsulation of the OEL device at the above deposition temperature.